1. Field of the Invention
The invention relates to a vehicle and a control method for the vehicle and, more particularly, to a vehicle that travels by using power of at least any one of an engine and a motor and a control method for the vehicle.
2. Description of Related Art
In recent years, a hybrid vehicle that travels by using power of at least any one of an engine and a motor is in widespread use. Some hybrid vehicles include a generator in addition to the motor. The generator generates electric power by using the power of the engine.
Japanese Patent Application Publication No. 2007-196733 (JP 2007-196733 A) describes a technique for, in a hybrid vehicle that includes an engine, a motor and a generator, executing control (hereinafter, also referred to as “battery-less travel control”) in which, at the time of a failure of a battery that stores electric power for driving the motor, the battery is isolated from an electrical system that includes the motor and the generator and then the motor is driven by using electric power generated by the generator with the use of the power of the engine. In addition, Japanese Patent Application Publication No. 2008-72868 (JP 2008-72868 A), Japanese Patent Application Publication No. 2007-137373 (JP 2007-137373 A) and Japanese Patent Application Publication No. 2009-45946 (JP 2009-45946 A) also describe related arts.
Incidentally, an inverter for driving the motor is mounted on a hybrid vehicle. Major control methods for the inverter include a pulse width modulation (hereinafter, also referred to as “PWM”) control method and a rectangular wave voltage control (hereinafter, also simply referred to as “rectangular control”) method. Rectangular control has a larger modulation factor (value corresponding to the percentage of an output voltage to an input voltage) of voltage conversion than PWM control and is able to increase a motor output, while it has a poor control accuracy (control response) and, therefore, an inverter output voltage tends to be unstable. Therefore, generally, rectangular control is used only in a high vehicle speed range, and PWM control is usually used.
On the other hand, during battery-less travel control, the battery cannot be used as an electric power buffer, and it is required to accurately keep the balance of input and output electric powers between the motor and the generator. However, when a vehicle speed falls in a high vehicle speed range during battery-less travel control, the control method for the inverter shifts from PWM control to rectangular control having poor control accuracy. By so doing, the balance of input and output electric powers is lost at the time when a steep change in required driving force has occurred, and a voltage (inverter output voltage) applied to the motor may be unstable.